1. Field of the Invention
The present invention relates generally to an optical network, and in particular, to an Ethernet passive optical network.
2. Description of the Related Art
A passive optical network is a subscriber network that forms a tree-structured distributed topology by connecting a plurality of optical network units (ONUs) to one optical line terminal (OLT) using a 1×N optical distribution network (ODN). The ITU-T (International Telecommunications Union—Telecommunication Standardization Sector) has recently published ITU-T G.982, ITU-T G.983.1 and ITU-T G.983.3, all of which describe an asynchronous transfer mode-passive optical network (ATM-PON) standard. In addition, IEEE 802.3ah TF organized by IEEE (Institute of Electrical and Electronics Engineers) has been working toward standardization for a gigabit Ethernet-based passive optical network system.
In ATM-PON and Ethernet passive optical network systems discussed in the standardization organizations such as ITU-T and IEEE 802.3, transmission capacity depends on a format of data carried on two different wavelengths between an OLT and an ONU. That is, the international standardization organizations of ITU-T and IEEE 802.3 have been discussing a transmission method in which an ATM cell or an Ethernet frame is carried on a 1,550 nm- or 1,490 nm-wavelength signal for downstream transmission from an OLT of the telephone office side to an ONU of the subscriber side, and data is carried on a 1,310 nm-wavelength signal for upstream transmission from an ONU of the subscriber side to an OLT of the telephone office side.
Standardization of a point-to-point gigabit Ethernet and a medium access control (MAC) technology for ATM-PON has already been completed and is disclosed in IEEE 802.3z and ITU-T G.983.1. MAC technology for ATM-PON is also discussed in U.S. Pat. No. 5,978,374, issued to Ghaibeh et al., on Nov. 2, 1999, entitled “Protocol for Data Communication over a Point-to-Multipoint Passive Optical Network.”
FIG. 1 illustrates a known configuration of an 802.1D bridge in an Ethernet system. Referring to FIG. 1, since, an Ethernet medium is a shared medium in accordance with 802.1D, an Ethernet frame transmitted from a subscriber terminal belonging to a particular region connected to a bridge is transmitted to all subscriber terminals in other regions connected to the bridge.
To reduce traffic and consequent collisions, the bridge can be subject to a table-based learning process; Upon receiving a frame, the bridge stores in an address table a source address of the received frame and an identifier of the port from which the frame was received. Through this process, the bridge progressively learns which addresses are assigned to respective ports. Upon receiving a frame after the learning, the bridge transmits the received frame only to the port assigned a destination address of the frame, and does not transmit the received frame to other ports, thereby mitigating the advent of collision in the shared medium.
If the bridge fails to detect a destination port, i.e., either the received frame has no destination or a port for a destination address of the received frame has never been learned, the bridge transmits the received frame to all ports except a source port from which the frame is received. Transmission to the source port is unnecessary, since terminals on the source port have already received the frame on a “broadcast and select” basis in light of characteristics of the Ethernet. Likewise, if the address table has a destination port for the received frame, but the destination port is the same as the source port, the bridge discards the frame. Thus, for example, the bridge in FIG. 1 discards an Ethernet frame from subscriber terminal A to subscriber terminal B since both terminals belong to the same region and are therefore connected to the bridge by the same port.
FIG. 2 illustrates a known Ethernet—passive optical network with an 802.1D L2 (Layer 2) switch, which can, in conjunction with a L3 (Layer 3) router, be utilized in place of an 802.1D bridge according to the prior art, and FIG. 3 illustrates a format of a standard Ethernet frame. The Ethernet passive optical network includes an optical line terminal (OLT) 110, an optical distribution network (ODN) 120, and optical network units (ONUs) 131-133.
The OLT 110 accesses data of each of the ONUs 131-133 by time division multiplexing (TDM). The OLT 110 includes an L2 switch, and transmits frames received from the ONUs 131-133 by matching corresponding addresses to corresponding ports. In a learning process that occurs during initialization, the L2 switch learns MAC addresses assigned to respective ports while storing in an address table a source address (SA) of a frame received at a particular port. In this case, MAC addresses of the ONUs 131-133 are used as the source addresses or the destination addresses (DA). Upon receiving a frame after the address learning process, the L2 switch transmits the received frame only to the port where a destination address of the frame is assigned, using a static filtering entry (see IEEE 802.1d clause 7.9.1).
As with the bridge of the previous example, the switch discards any frame whose source port is identical to its destination port. Therefore, any frame whose destination port is identical to its source port is discarded by the bridge or L2 switch without being transmitted downstream, even if that frame must be subject to upstream transmission to the bridge or L2 switch and subsequent downstream transmission through the same port.